Armor plate, sheet, and bar are commonly provided to protect structures against forcibly launched projectiles. Although armor plate, sheet, and bar are typically used in military applications as a means to protect personnel and property within, for example, vehicles and mechanized armaments, the products also have various civilian uses. Such uses may include, for example, sheathing for armored civilian vehicles and blast-fortified property enclosures. Armor has been produced from a variety of materials including, for example, polymers, ceramics, and metallic alloys. Because armor is often mounted on mobile articles, armor weight is typically an important factor. Also, the costs associated with producing armor can be substantial, and particularly so in connection with exotic armor alloys, ceramics, and specialty polymers. As such, an objective has been to provide lower-cost yet effective alternatives to existing armors, and without significantly increasing the weight of armor necessary to achieve the desired level of ballistic performance (penetration resistance and cracking resistance).
Also, in response to ever-increasing anti-armor threats, the United States military had for many years been increasing the amount of armor used on tanks and other combat vehicles, resulting in significantly increased vehicle weight. Continuing such a trend could drastically adversely affect transportability, portable bridge-crossing capability, and maneuverability of armored combat vehicles. Within the past decade the U.S. military has adopted a strategy to be able to very quickly mobilize its combat vehicles and other armored assets to any region in the world as the need may arise. Thus, concern over increasing combat vehicle weight has taken center stage. As such, the U.S. military has been investigating a number of possible alternative, lighter-weight armor materials, such as certain titanium alloys, ceramics, and hybrid ceramic tile/polymer-matrix composites (PMCs).
Examples of common titanium alloy armors include Ti-6Al-4V, Ti-6Al-4V ELI, and Ti-4Al-2.5V—Fe—O. Titanium alloys offer many advantages relative to more conventional rolled homogenous steel armor. Titanium alloys have a high mass efficiency compared with rolled homogenous steel and aluminum alloys across a broad spectrum of ballistic threats, and also provide favorable multi-hit ballistic penetration resistance capability. Titanium alloys also exhibit generally higher strength-to-weight ratios, as well as substantial corrosion resistance, typically resulting in lower asset maintenance costs. Titanium alloys may be readily fabricated in existing production facilities, and titanium scrap and mill revert can be remelted and recycled on a commercial scale. Nevertheless, titanium alloys do have disadvantages. For example, a spall liner typically is required, and the costs associated with manufacturing the titanium armor plate and fabricating products from the material (for example, machining and welding costs) are substantially higher than for rolled homogenous steel armors.
Although PMCs offer some advantages (for example, freedom from spalling against chemical threats, quieter operator environment, and high mass efficiency against ball and fragment ballistic threats), they also suffer from a number of disadvantages. For example, the cost of fabricating PMC components is high compared with the cost for fabricating components from rolled homogenous steel or titanium alloys, and PMCs cannot readily be fabricated in existing production facilities. Also, non-destructive testing of PMC materials may not be as well advanced as for testing of alloy armors. Moreover, multi-hit ballistic penetration resistance capability and automotive load-bearing capacity of PMCs can be adversely affected by structural changes that occur as the result of an initial projectile strike. In addition, there may be a fire and fume hazard to occupants in the interior of combat vehicles covered with PMC armor, and PMC commercial manufacturing and recycling capabilities are not well established.
Metallic alloys are often the material of choice when selecting an armor material. Metallic alloys offer substantial multi-hit protection, typically are inexpensive to produce relative to exotic ceramics, polymers, and composites, and may be readily fabricated into components for armored combat vehicles and mobile armament systems. It is conventionally believed that it is advantageous to use materials having very high hardnesses in armor applications because projectiles are more likely to fragment when impacting higher hardness materials. Certain metallic alloys used in armor application may be readily processed to high hardnesses, typically by quenching the alloys from very high temperatures.
Because rolled homogenous steel alloys are generally less expensive than titanium alloys, substantial effort has focused on modifying the composition and processing of existing rolled homogenous steels used in armor applications since even incremental improvements in ballistic performance are significant. For example, improved ballistic threat performance can allow for reduced armor plating thicknesses without loss of function, thereby reducing the overall weight of an armor system. Because high system weight is a primary drawback of metallic alloy systems relative to, for example, polymer and ceramic armors, improving ballistic threat performance can make alloy armors more competitive relative to exotic armor systems.
Over the last 25 years, relatively light-weight clad and composite steel armors have been developed. Certain of these composite armors, for example, combine a front-facing layer of high-hardness steel metallurgically bonded to a tough, penetration resistant steel base layer. The high-hardness steel layer is intended to break up the projectile, while the tough underlayer is intended to prevent the armor from cracking, shattering, or spalling. Conventional methods of forming a composite armor of this type include roll bonding stacked plates of the two steel types. One example of a composite armor is K12® armor plate, which is a dual hardness, roll-bonded composite armor plate available from ATI Allegheny Ludlum, Pittsburgh, Pa. K12® armor plate includes a high hardness front side and a softer back side. Both faces of the K12® armor plate are Ni—Mo—Cr alloy steel, but the front side includes higher carbon content than the back side. K12® armor plate has superior ballistic performance properties compared to conventional homogenous armor plate and meets or exceeds the ballistic requirements for numerous government, military, and civilian armoring applications. Although clad and composite steel armors offer numerous advantages, the additional processing involved in the cladding or roll bonding process necessarily increases the cost of the armor systems.
Relatively inexpensive low alloy content steels also are used in certain armor applications. As a result of alloying with carbon, chromium, molybdenum, and other elements, and the use of appropriate heating, quenching, and tempering steps, certain low alloy steel armors can be produced with very high hardness properties, greater than 550 BHN. Such high hardness steels are commonly known as “600 BHN” steels. Table 1 provides reported compositions and mechanical properties for several examples of available 600 BHN steels used in armor applications. MARS 300 and MARS 300 Ni+ are produced by the French company Arcelor. ARMOX 600T armor is available from SSAB Oxelosund AB, Sweden. Although the high hardness of 600 BHN steel armors is very effective at breaking up or flattening projectiles, a significant disadvantage of these steels is that they tend be rather brittle and readily crack when ballistic tested against, for example, armor piercing projectiles. Cracking of the materials can be problematic to providing multi-hit ballistic resistance capability.
TABLE 1YieldTensilePSStrengthStrengthElong.BHNAlloyCMn(max)(max)SiCrNiMo(Mpa)(Mpa)(%)(min)Mars0.45-0.550.3-0.70.0120.0050.6-1.00.44.50.3-0.5≧1,300≧2,000≧6%578-655300(max)(max)Mars0.45-0.550.3-0.70.010.0050.6-1.00.01-0.043.5-4.50.3-0.5≧1,300≧2,000≧6%578-655300Ni+Armox0.471.00.0100.0050.1-0.71.53.00.7 1,500 2,000≧7%570-640600(max)(max)(max)(max)(max)(typical)(typical)
In light of the foregoing, it would be advantageous to provide an improved steel armor material having hardness within the 600 BHN range and having substantial multi-hit ballistic resistance with reduced crack propagation.